Passive star couplers are widely utilized in fiber optic networks for distributing light (i.e., optical power) from one group of optical fiber links to another. The distribution function is a parallel process, whereby optical power injected into any one of the inputs of the coupler is distributed in parallel to all of its outputs. As a general rule, these couplers have dedicated inputs and outputs because they usually are employed in networks which superimpose a predetermined directionality upon them. However, there are passive star couplers which have generally symmetrical optical characteristics, so they may also be used to bidirectionally distribute optical power in the absence of any such external constraints.
Some passive star optical couplers, such as those shown in U.S. Pat. No. 3,883,217, have a mixing rod for distributing light from a group of input fibers to a separate, physically distinct group of output fibers. These couplers typically have relatively uniform optical power distribution characteristics, whereby light injected into any of the input fibers of such a coupler is more or less equally divided among all of its output fibers. However, such couplers often cause significant attenuation of the injected light because of their insertion losses and their so-called "packing fraction" losses. See, Rawson E. G. and Metcalfe, R. M., "Fibernet: Multimode Optical Fibers for Local Computer Networks," IEEE Transactions on Communications, Vol. COM-26, No. 7, pp. 983-990. As Rawson and Metcalfe point out, supra at p. 986, steps can be taken to improve the efficiency of mixing rod couplers, but the cost of doing so often outweighs the increased efficiencies that can be achieved.
Accordingly, biconically tapered star couplers, such as described in Rawson, E. G. and Bailey, M. D., "Bitaper Star Coupler with up to 100 Fibre Channels," Electronic Letters, Vol. 15, No. 14, July 5, 1979, pp. 432-433, have become the passive star couplers of choice for most applications requiring relatively efficient (i.e., low loss) distribution of the available optical power. A preferred technique for fabricating such a coupler is to locally heat intermediate segments of a plurality of tensioned and coaxially twisted optical fibers, so that the claddings of the heated fiber segments soften sufficiently to thermally fuse and optically unit while the fibers are being drawn to form an axially extending, biconical taper. See, E. G. Rawson and A. B. Nafarrate, Electronics Letters, Vol. 14, No. 9, Apr. 27, 1978, pp. 274-275. As is known, light injected into any one of the fibers of such a coupler is gradually transformed into all of the outer fibers as the light passes through the biconically tapered segment. Characteristically, of course, the input and output ports of such a coupler are the opposite ends of the same fibers, rather than being defined by physically distinct fibers.
Known biconically tapered star couplers suffer from the disadvantage of having one output receive a disproportionately large share of the available optical power. Specifically, light injected into any one of the inputs of such a coupler is relatively uniformly distributed to all of its outputs, with the single exception of the one which is defined by the excited or illuminated fiber. Analysis indicates that only about 90%-95% of the injected power is extracted from the excited fiber, which suggests that the excited fiber has an elevated output power level because it not only receives its pro rata share of the extracted power, but also as much as an additional 5%-10% of the injected power. That conclusion is supported by a finding that the ratio of the output power of the excited or so-called "bright fiber" to the average output level of the other, non-excited fibers increases as a function of the number of fibers, N, contained by the coupler. Existing biconically tapered couplers have dynamic output power ranges of about 2:1 to about 10:1, but even wider dynamic output power ranges can be anticipated if conventional biconically tapered coupler technology is applied to satisfy the demand for couplers having increased channel capacities.
It has been found that the imperfect power extraction mechanism is the most significant cause of the non-uniform optical power distribution characteristics of both large and small biconically tapered couplers. Although the normalized standard deviation of the output power levels of the non-excited fibers of such a coupler relative to average output power level of such fibers also increases as a function of N, it characteristically ranges from approximately 7%-17% for relatively large couplers containing one hundred fibers or so, and is even less for smaller couplers.
Heretofore, system designers have made allowances for the wide dynamic output power ranges of biconically tapered couplers. It has been recognized that the bright fiber of such a coupler cannot be identified in advance, so the accepted practice is to design the system components (e.g., receivers) which are to be driven by the outputs of such a coupler to accomodate the "worst case" dynamic output power range of the coupler, even though that necessarily increases the cost and complexity of those components. Moreover, there has been substantial reluctance to employ such couplers for applications in which significant output power variations cannot be tolerated.
Others have attempted to reduce the dynamic output power range of biconically tapered couplers. For example, it has been suggested that the biconical taper of such a coupler can be temporally severed at its waist, so that the severed segments of the taper can be re-fused after they have been rotated relative to one another as required to more uniformally distribute injected power to all of the outputs of the coupler. These additional manufacturing steps necessarily increase the cost and complexity of the coupler fabrication process, and do not fully compensate for the imperfect power extraction mechanism of the central fibers of such couplers. Similarly, it has been suggested that a mixing rod can be fused between the severed segments of the biconical taper of such a coupler, so that the mixing rod relatively uniformally distributes the injected power to all of the coupler outputs. This approach essentially eliminates the "bright fiber," but it increases the cost and complexity of both the coupler and the coupler fabrication process. Moreover, both of the above described proposals increase the coupler insertion loss by introducing fusion losses.